Pardon a very newbie question, but is there a public domain tool
available (besides VCG) that can directly display the structure of a
grammar from a yacc or ebnf description? The problem with VCG, as far
as I can see so far, is that one has to write a grammar-to-graph
translator before one could use it for this application.

I'm particularly interested in such a tool for visually comparing
different specifications of the same language, so a tool that
permitted side-by-side or multiple-window comparisons of different
grammars would be very helpful.

Either Win95 or linux/X11 implementations would be the most useful to
me, with a preference for Win95 at the moment.